Posts from October 2012

  • On Algorithms

    So, generally speaking, I’ve typically adhered to the rule that those who develop software should be aware of various classes of algorithms and data structures, but should avoid implementing them if at all possible. The reasoning here is pretty simple, and I think pretty common:

    1. You’re reinventing the wheel. Stop that, we have enough wheels.
    2. You’re probably reinventing it badly.

    So just go find yourself the appropriate wheel to solve your problem and move on.

    Ah, but there’s a gotcha, here: Speaking for myself, I never truly understand an algorithm or data structure, both theoretically (ie, how it works in the abstract, complexity, etc) and practically (ie, how you’d actually implement the thing) until I try to implement it. After all, these things in the abstract can be tricky to grok, and when actually implemented you discover there’s all kinds of details and edge cases that you need to deal with.

    Now, I’ve spent a lot of my free time learning about programming languages (the tools of our trade that we use to express our ideas), and about software architecture and design, the “blueprints”, if you will. But if languages are the tools and the architecture and design are the blueprints, algorithms and data structures are akin to the templates carpenters use for building doors, windows, etc. That is, they provide a general framework for solving various classes of problems that we as developers encounter day-to-day.

    And, like a framer, day-to-day we may very well make use of various prefabbed components to get our jobs done more quickly and efficiently. But without understanding how and why those components are built the way they are, it can be very easy to misuse or abuse them. Plus, it can’t hurt if, when someone comes along and asks you to show off your mad skillz, you can demonstrate your ability to build one of those components from scratch.

    Consequently, I plan to kick off a round of posts wherein I explore various interesting algorithms and data structures that happen to catch my attention. So far I have a couple on the list that look interesting, either because I don’t know them, or because it’s been so long that I’ve forgotten them…

    Data Structures

    1. Skip list
    2. Fibonacci heap
    3. Red-Black tree
    4. Tries
      1. Radix/PATRICIA Tries
      2. Suffix Tries
    5. Bloom filter


    1. Various streaming algorithms (computations over read-once streams of data):
      1. Heavy hitters (finding elements that appear more often than a proscribed freqency)
      2. Counting distinct elements
      3. Computing entropy
    2. Topological sort

    And I guarantee there’s more that belong on this list, but this is just an initial roadmap… assuming I follow through, anyway.

  • Hosting Git on Windows

    Using Git to push changes upstream to servers is incredibly handy. In essence, you set up a bare repository on the target server, configure git to use the production application path as the git working directory, and then set up hooks to automatically update the working directory when changes are pushed into the repository. The result is dead easy code deployment, as you can simply push from your repository to the remote on the server.

    But making this work when the Git repository is being hosted on Windows is a bit tricky. Normally ssh is the default transport for git, but making that work on Windows is an enormous pain. As such, this little writeup assumes the use of HTTP as the transport protocol.


    So, first up we need to install a couple components:

    1. msysgit
    2. Apache

    Note: When installing msysgit, make sure to select the option that installs git in your path! After installation the system path should include the following1:

    C:\Program Files\Git\cmd;C:\Program Files\Git\bin;C:\Program Files\Git\libexec\git-core

    Now, in addition, we’ll be using git-http-backend to serve up our repository, and it turns out the msysgit installation of this tool is broken such that one of its required DLLs is not in the directory where it’s installed. As such, you need to copy:

    C:\Program Files\Git\bin\libiconv-2.dll


    C:\Program Files\Git\libexec\git-core\

    Repository Initialization

    Once you have the software installed, create your bare repository by firing up Git Bash and running something like:

    $ mkdir -p /c/git/project.git
    $ cd /c/git/project.git
    $ git init --bare
    $ git config core.worktree c:/path/to/webroot
    $ git config http.receivepack true
    $ touch git-daemon-export-ok

    Those last three commands are vital and will ensure that we can push to the repository, and that the repository uses our web root as the working tree.

    Configuring Apache

    Next up, add the following lines to your httpd.conf:

    SetEnv GIT_PROJECT_ROOT c:*git*
    ScriptAlias *git* "C:/Program Files/Git/libexec/git-core/git-http-backend.exe/"
    <Directory "C:/Program Files/Git/libexec/git-core/">
      Options +ExecCGI FollowSymLinks
      Allow From All

    Note, I’ve omitted any security, here. You’ll probably want to enable some form of HTTP authentication.

    In addition, in order to make hooks work, you need to reconfigure the Apache daemon to run as a normal user. Obviously this user should have permissions to read from/write to the git repository folder and web root.

    Oh, and last but not least, don’t forget to restart Apache at this point.

    Pushing the Base Repository

    So, we now have our repository exposed, let’s try to push to it. Assuming you have an already established repository ready to go and it’s our master branch we want to publish, we just need to do a:

    git remote add server http://myserver/git/project.git
    git push server master

    In theory, anyway.

    Note: After the initial push, in at least one instance I’ve found that “logs/refs” wasn’t present in the server bare repository. This breaks, among other things, git stash. To remedy this I simply created that folder manually.

    Lastly, you can pop over to your server, fire up Git Bash, and:

    $ cd /c/git/project.git
    $ git checkout master

    Our Hooks

    So, about those hooks. I use two, one that triggers before a new update comes to stash any local changes, and then another after a pack is applied to update the working tree and then unstash those local changes. The first is a pre-receive hook:

    export GIT_DIR=`pwd`
    cd `git config --get core.worktree`
    git stash save --include-untracked

    The second is a post-update hook:

    export GIT_DIR=`pwd`
    cd `git config --get core.worktree`
    git checkout -f
    git reset --hard HEAD
    git stash pop

    Obviously you can do whatever you want, here. This is just something I slapped together for a test server I was working with.

    1. Obviously any paths, here, would need to be tweaked on a 64-bit server with a 32-bit Git.